The invention is in the field of electronic reproduction technology and is directed to a method and to an apparatus for the elimination of light by absorption.
The method and the apparatus are employed, for example, in an electronic recording device for point-by-point and line-by-line exposure of a recording medium with a laser beam.
In a recording device, also called an exposer or recorder, a laser beam modulated by a video signal is conducted point-by-point and line-by-line across a recording material to be exposed. The recording material is thereby fixed on a holder that moves relative to the light beam. In the case of an inside-drum recording device, the recording material is fixed on a stationary holder or exposure trough shaped like a cylindrical segment, and the laser beam is conducted across the recording material point-by-point and line-by-line with a rotating light beam deflection unit. The recording device, however, can also be designed as a drum device or flat bed device.
For direct exposure of printing plates, a laser beam having a high light power is required, this being generated in a laser light source for example in the form of a YAG laser. The modulation of the laser beam by the video signal occurs in a video modulator that, for example, is designed as an acousto-optical modulator (AOM). The acousto-optical modulator uses the input laser beam to generate essentially a used output laser beam of the first order, referred to below as an illumination beam, and an unused output laser beam of the 0 order, referred to below as a secondary beam.
While the illumination beam is being conducted across the recording material point-by-point and line-by-line with the rotating light beam deflection unit, the undesired secondary beam must be absorbed in a device, called a light sump or a light trap (beam dump).
There is a similar problem when a controlled light beam deflector that deflects the light beam employed for the illumination into the light trap during the illumination pauses is employed instead of the video modulator.
In order to achieve a good recording quality given the recording device, back reflections of light into the laser light source must be avoided, since they excite disturbing oscillations in the laser light source. Such back reflections can be caused directly by the unused secondary beam or by reflections of the secondary beam at optical components and housing parts, being caused as stray light when the light power of the undesired secondary beam is not completely and designationally eliminated in the light trap. Given the traditional light trap in the form of an absorption block, back reflections in the laser light source cannot, however, be completely avoided, so that the recording quality can be deteriorated.
EP 0 533 346 A and U.S. Pat No. 5,457,567 A already disclose laser recording devices wherein an illumination beam used for registration and an unused secondary beam are generated from an input laser beam in an acousto-optical modulator (AOM), and wherein the unwanted secondary beam is absorbed in a light trap. No specific exemplary embodiments for such light traps are recited in these publications.
Patent Abstracts of Japan, vol. 013, no. 407 (E-818), Sep. 8, 1989 and JP 01 146386 A (Toshiba Corp.), Jun. 8, 1989, already disclose a light trap that is composed of a cylindrical hollow body, whose one face side forms the light entry aperture, and of an axially symmetrically arranged, conically shaped reflector.
However, these known light traps do not allow back reflections into the laser light source of a laser recording device to be adequately avoided, so that the recording quality can be deteriorated.